New Zealand sea lion numbers are declining because they're getting caught in fishing nets, according to Otago University-led research.
The accidental catch is occurring despite the use of exclusion devices - an attachment in the net to allow sea lions to escape - on sub-Antarctic squid trawlers for the past 16 years.
The researchers, also from Massey and Toronto universities, believe the current management of sea lion bycatch around the Auckland Islands places the population at risk of extinction.
They say the reason is the government assumes fishing is not a major threat to the species.
In the study, published in the journal Proceedings of the National Academy of Sciences, the scientists looked at government data of the sea lion population and fisheries bycatch.
They say the figures indicate exclusion devices have paradoxically contributed to ongoing decline rather than recovery.
The devices could be obscuring sea lion deaths by allowing dead animals to fall out of the nets, or causing injury that reduces life expectancy or reproductive ability.
Lead author Dr Stefan Meyer says several threats, such as disease and bycatch, have been postulated as causes of the drop in sea lion numbers.
Until the present research, he says, studies have been unable to link these threats to the decline.
"We now know that sea lion exclusion devices have, despite all assumptions, obscured bycatch of New Zealand sea lions and that this factor posed a significant and ongoing impact to the population," he said.
Associate Professor Bruce Robertson said the researchers hoped the study would lead to meaningful management.
He said there were a range of options open to the government that would still allow commercial fishing in the New Zealand sub-Antarctic.